yups, have a look at the specs on the celestion website.
A V30 weights 4.7 kg's
A G12T-75 only 3.5 kg's
the voice coil diameter seems to be the same, both 44.5 mm
but anyhow, i don't think the weight of the wood is that important, if they made it more solid, better bracing etc... it would have an effect on the sound too, and using a grill vs a cloth before the speakers should have it's influence too.µ
i beleave Mustaine had those custom cabs for quite a while now. i remember having seen those on stage.